1. Field of the Invention
The present invention relates to a pressure monitoring system for monitoring a hydraulic pressure accumulated in an accumulator, and more particularly to the system for monitoring the hydraulic pressure which is supplied from a fluid pump and accumulated in the accumulator, for use in a vehicle anti-skid control system, for example.
2. Description of the Related Arts
In a prior anti-skid control system for an automotive vehicle, a pressure sensor and a pressure switch have been used for monitoring a hydraulic pressure which is supplied from a fluid pump and accumulated in an accumulator, as disclosed in a service manual of 1991 model of SOARER manufactured by Toyota Motor Corporation, pages 3-150 to 152, for example. This manual discloses output characteristics of the pressure sensor and pressure switch, and the operation of the motor or the like activated in response to output signals of the sensor and switch.
The pressure sensor includes a metallic diaphragm to which a hydraulic pressure is applied, and a semiconductor strain gauge which transforms the strain of the diaphragm into electric signals. The signals include a first switching signal for controlling a signal to actuate the motor, and a second switching signal for detecting an excessive pressure decrease. When the hydraulic pressure in the accumulator is decreased to be lower than a predetermined intermediate pressure, the pressure sensor provides the first switching signal as L (low) level to activate the motor. Whereas, when the pressure in the accumulator is increased to be higher than a predetermined high pressure, the pressure sensor provides the first switching signal as H (high) level to stop the operation of the motor. If the accumulator is decreased to be lower than a predetermined low pressure, i.e., the excessive low pressure, the pressure sensor provides the second switching signal as L (low) level to turn on a brake warning lamp, and to activate a buzzer. Thus, the first switching signal has a hysteresis as described above, so does the second switching signal.
On the contrary, the pressure switch includes a rubber diaphragm to which the hydraulic pressure is applied, and a micro-switch which is actuated by the diaphragm through a rod and a spring in response to displacement of the diaphragm. The pressure switch is provided for redundancy to the pressure sensor. That is, if a malfunction of the pressure sensor occurs, the pressure switch is used for controlling the operation of the motor. Therefore, it is so arranged that if the accumulator is decreased to be lower than a predetermined pressure approximately equal to the excessive low pressure, the pressure switch is activated to turn on the micro-switch to provide a H level signal, whereas the pressure switch is held to be off to keep the micro-switch off so as to provide a L level signal, when the hydraulic pressure in the accumulator is higher than a predetermined pressure which is set to be higher than the excessive low pressure by a certain range.
Comparing with the pressure switch, however, the pressure sensor is much higher in price, so that it causes cost-up in the anti-skid control system as a whole.